The present invention relates to a digital multiplexer for PCM voice communication and, more particularly, a digital multiplexer which operates to multiplex PCM voice channels on DS1, DS1C and DS2 digital transmission lines into one or more DS3 transmission lines and vice versa.
The standard for digital multiplexers operating to multiplex DS1, DS1C and DS2 transmission lines into a DS3 transmission line are set forth and discussed in the Bell System Transmission Engineering Technical Reference entitled "Digital Multiplexes, Requirements and Objectives" by the Director, Exchange Systems Design, A T & T (July, 1982). Digital multiplexers which are connected into the Bell System pulse code modulated (PCM) voice signal network must conform with this standard.
As is well known, a single PCM voice channel, known as a "DSO" channel, operates at 64 kilobits per second (Kb/sec) to transmit 8,000 8-bit voice samples per second. According to the Bell standard, individual voice channels are multiplexed into higher speed channels for long distance transmission. As a particular example, 24 DSO channels may be multiplexed into a "DS1" channel operating at 1.544 Mb/sec. ln this format, 24 8-bit samples, one from each DS0 channel, are arranged serially in a single transmission frame together with a framing bit to form a 193-bit frame. Transmission of successive 193-bit frames at a rate of 8,000 frames per second determines the bit rate of 1.544 Mb/sec. Set forth in the following table are the Bell standard digital transmission lines with their associated transmission rates and numbers of voice channels:
TABLE ______________________________________ Number of Transmission Line Voice Channels Transmission Rate ______________________________________ DS0 1 64 Kb/sec. DS1 24 Approx. 1.5 Mb/sec. .sup. DS1C 48 Approx. 3 Mb/sec. DS2 96 Approx. 6 Mb/sec. DS3 672 Approx. 45 Mb/sec. ______________________________________
FIGS. 1-3 of the drawings depict the structure and nomenclature of conventional, state-of-the-art equipment for connecting together digital PCM voice transmission lines having different transmission rates. FIG. 1 shows a so-called "M13" multiplexer which multiplexes 28 DS1 transmission lines into a single DS3 transmission line. As is shown, this is accomplished by providing seven M12 multiplexers, each of which multiplexes four DS1 lines into a single DS2 line, and providing a single M23 multiplexer which multiplexes seven DS2 lines into a single DS3 line.
Additional flexibility is provided by a so-called "MX3" multiplexer which is represented in FIG. 2. The MX3 multiplexer is capable of connecting different numbers of DS1, DS1C and DS2 lines to a single DS3 line. As is shown in this particular example, the seven DS2 lines connected to the M23 multiplexer are formed by concentrating two groups of four DS1 lines, concentrating two groups of two DS1C lines and by direct connection to three DS2 lines. As will be appreciated, the "X" in the MX3 multiplexer designation refers to the fact that three different types of transmission lines--namely DS1, DS1C and DS2--are connectable to this device.
FIG. 3 illustrates a complete switching "node" which connects a variable number of DS1, DS1C, DS2 aad DS3 lines as inputs and outputs and routes any voice channel on any one line to a voice channel on any other line. This is accomplished with the aid of a digital cross-connect system (DCS) which operates in the manner of a telephone exchange to connect any incoming transmission line to any outgoing transmission line. One such digital cross-connect system is currently marketed by Western Electric under the acronym "DACS" (Digital Access and Cross-Connect System). This known cross-connect system provides a crosspoint array to enable any transmission line of one transmission rate to be connected to any other transmission line of like transmission rate.
Conversion from one transmission rate to another is effected by a plurality of MX3 multiplexers. M12 and MC2 multiplexers (not shown) may also be provided, as desired, to convert to a DS2 transmission rate.
While equipment of the type illustrated in FIG. 3 may be tailored to voice transmission network nodes of any configuration, such equipment is not readily programmable so that changes in the node configuration are expensive and time consuming. This equipment includes certain dedicated multiplexer hardware which must be physically interconnected into or removed from the system each time a change is made.